1. Field of the Invention
The present invention relates to a recording apparatus for recording images by utilizing thermal energy, a recording head unit mounted on the recording apparatus, and a temperature controller for use in the recording head.
In particular, it relates to a recording apparatus provided with a heat generating resistive element for generating thermal energy and carrying out image formation by discharging the ink droplet by usage of heat generation of the element, a recording head unit provided with the above recording apparatus, and temperature controlling apparatus used for the above head unit.
2. Related Background Art
As one of recording systems for producing images on a material on which the images are to be recorded, there is known an ink jet recording system in which ink is discharged in the form of droplets for recording.
Recording heads combined with such an ink jet recording system have widely been employed in recording output units for computers, word processors and the like because of lower operating noise, a simple basic mechanical structure, and lower cost than other recording systems.
Generally, a recording head in the ink jet recording system includes discharge energy generator means which are each disposed at a discharge port or orifice (opening) for discharging ink in the form of droplets, or at a part of a liquid passage or liquid chamber communicating with the discharge port, thereby imparting discharge energy sufficient to discharge a part of ink within the liquid passage to form a flying droplet.
Known discharge energy generator means include, for example, mechanical energy generator means such as electro-mechanical transducers represented by piezoelectric elements, electromagnetic energy generator means for irradiating electromagnetic waves such as lasers to ink that absorbs the irradiated energy to form flying droplets, and thermal energy generator means such as electro-thermal transducers.
Those ink jet recording heads which employ thermal energy generator means such as electro-thermal transducers, in particular, among the above energy generator means, can be manufactured relatively simply by making use of the microprocessing technology which has recently gained remarkable progress in the field of semiconductors. Thus, the microprocessing technology makes it possible to easily manufacture recording heads in long sizes or in a planar or two-dimensional form, so that discharge ports can be formed in the multi-orifice arrangement with higher density.
As a result, it has become easy to produce recording with higher resolution and fabricate recording heads in compact sizes.
FIG. 1 is a schematic perspective view showing one example of an ink jet recording head in which 10 electro-thermal transducers are used as the thermal energy generator means. As seen from FIG. 1, a recording head 2 mainly comprises electro-thermal transducers 51, electrodes 52, liquid passage walls 53 and a top plate 54. The recording head is formed on a silicon substrate 50 through semiconductor manufacture processes such as etching, vaporing or sputtering. Ink 13 is supplied from an ink reservoir (not shown) through an ink supply tube 55 to a common liquid chamber 56 of the recording head 2. Denoted by 57 is a connector for the ink supply tube 55, and 60 is an ink supply port.
The ink 13 is supplied by a capillary phenomenon from the ink reservoir to the common liquid chamber 56 and then to respective liquid passages 10 for forming a meniscus at each ink discharge port 59, defined at the distal end of the liquid passage 10, in a stable state. When the electro-thermal transducer 51 is energized to generate heat, this abrupt heating causes the ink 13 to develop a film boiling phenomenon to produce vapor bubbles in the ink 13. Dependent on expansion and contraction of the produced bubbles, the ink is discharged from the ink discharge ports 39 to form flying droplets.
With the above arrangement of the recording head, not only ink jet recording heads of the multi-nozzle type having 128 or 256 nozzles, but also longer recording heads for full-line printers with nozzles in numbers ranging from one thousand and several hundreds to several thousands can easily be manufactured with high productivity through achievement of nozzle arrangements of high nozzle density, e.g., 16 nozzles/mm.
In those ink jet recording heads using the thermal energy generator means such as electro-thermal transducers, however, since ink is discharged under the generation of heat by the electro-thermal transducers, the temperature of the discharged ink is raised with repeated cycles of heating during the recording process. As a result, the formed droplets might be affected adversely to degrade the recording quality of images.
The above technical problem will be described below in more detail. Assuming that the voltage and current applied to the electro-thermal transducers are held constant, changes in temperature of a recording head is related to the amount of ink per dot discharged from the recording head such that as the temperature of the discharged ink rises, viscosity of ink is reduced and an amount of ink per dot is increased.
Dependent on changes in the temperature of the discharged ink, therefore, dots deposited on the surface of recording paper to form an image become different in the size and eventually the intended recording image density or tone cannot be obtained.
In an ink jet recording apparatus comprising a plurality of recording heads, the temperatures of the discharged ink may be different from one another with the lapse of recording time due to a difference in the frequency of use between the respective heads. Thus, supposing the diameter of recording dot to be D.sub.0 at the record lapse time of zero under a certain temperature of the recording heads, for example, one recording head has a temperature T.sub.A (.degree. C.) and the other recording head has a temperature T.sub.B (.degree. C.) at the record lapse time of t, whereby the recording heads respectively produce the diameters D.sub.A, D.sub.B of recording dots different from each other. This results in a problem to be solved that an image recorded by the plurality of recording heads is out of density balance in the monochromatic case, or out of color balance when the recording heads respectively discharge ink of different colors.
Meanwhile, when the recording apparatus has not been used for a long period of time, the entire apparatus including the head and the ink supply system becomes substantially equal to an environmental temperature of the surroundings.
In such a case, if the environmental temperature is low, the ink temperature is also reduced and hence viscosity of ink is increased to prevent the recording apparatus from quickly starting operation at the time of start-up. As a result, the recording apparatus might require a long period of time until desired recording is achieved, with the result that it can not be driven immediately after turning it on.
Thus, in the recording apparatus that employs ink, changes in the ink temperature vary viscosity of the ink or produce dissolved gas in the ink. Stated otherwise, ink discharge characteristics are largely affected by not only the ink temperature but also the temperature in the vicinity of a recording head surrounding the ink.
As described above, the discharged state of ink is greatly dependent on such factors as viscosity of the ink in the discharge port or orifice, the generation process and maximum volume of bubbles caused by a phase change of the ink upon thermal energy being applied thereto. Then, changes in viscosity of the ink is dependent on a length of time in which recording has not been made, and the ink temperature. The generation process and maximum volume of bubbles are also mainly dependent on the ink temperature. Accordingly, control of the ink temperature is a basic factor for maintaining the performance of ink jet recording. The following has therefore been proposed in the past as means to stabilize ink characteristics.
First, Japanese Patent Publication No. 53-42619, No. 55-5429 and the like disclose an arrangement that an ink supply passage is associated at its part with an ink warming device, and ink is always warmed to keep the ink temperature constant. Secondly, Japanese Patent Laid-Open No. 50-4912 discloses an arrangement that a head is provided on its rear surface with a heat conducting member and a heater to warm an ink supply passage for keeping the ink temperature constant.
Thirdly, Japanese Patent Publication No. 55-6509 discloses an arrangement that a member of high thermal conductivity is brought into contact with a print head, which has been overheated, as required to accelerate heat radiation from the print head.
Fourthly, Japanese Patent Publication No. 56-9429 discloses an arrangement that a head housing is surrounded by a group of Peltier effect elements for purpose of quickly cooling the housing to reduce a pressure in an ink chamber.
With the first arrangement, however, although the temperature of ink flowing into the respective discharge ports can be held in certain range, the ink temperature becomes different between those discharge ports which are used for recording with higher frequency and those discharge ports which are used for recording with lower frequency. This makes it difficult to provide the uniform discharge performance throughout the head. With the second arrangement, since the ink flowing into the respective discharge ports is warmed regardless of frequency of use thereof, there still remains the similar problem as that in the above case.
With the third arrangement, the ink is prevented from being overheated. But, the above problem still remains in this case, too, because heat is radiated without considering frequency of use of the respective discharge ports and the temperature environment is differently developed for each of the discharge ports. With the fourth arrangement, the head housing is cooled to reduce a pressure in the ink chamber for purpose of preventing the ink from dripping from the discharge ports. However, since the temperature adjustment in this case is not carried out taking into account the fact that the discharge characteristics are dependent on viscosity of the ink, an improvement in recording characteristics has not resulted.
In the foregoing proposed arrangements, notwithstanding a single recording head contains an area where ink is discharged more frequently and an area where ink is discharged less frequently, the recording head is controlled to warm the whole of ink by applying a uniform amount of heat thereto, or to remove a uniform amount of heat from the entire head. As explained above, therefore, viscosity of the ink might be varied due to variations in the ink temperature between the respective discharge ports to render the discharged state of ink different therebetween, whereby the proper recording could not be ensured.
Particularly, in the case where recording paper of large width is used and a recording head itself is increased in its length, or where ink is discharged from the discharge orifices unevenly dependent on the content of recording, there still remains a problem that heat flux density in a recording head largely differs between head areas, and the temperature distribution is made not uniform in the single head.
In other words, with the recording head being increased in its length, it might be more difficult to heat or cool for holding the entire recording head at a uniform temperature, thereby causing temperature variations in the direction of length of the recording head.